Is there a good way to have a Map<String, ?> get and put ignoring case? [duplicate]

Question

This question already has an answer here:

• Case insensitive string as HashMap key 12 answers

Is there a good way to have a Map<String, ?> get and put ignoring case?

Answer 1

TreeMap extends Map and supports custom comparators.

String provides a default case insensitive comparator.

So:

final Map<String, ...> map = new TreeMap<>(String.CASE_INSENSITIVE_ORDER);

The comparator does not take locale into account. Read more about it in its JavaDoc.

Answer 2

You could use CaseInsensitiveMap from Apache's Commons Collections.

Answer 3

Would it be possible to implement your own Map overriding put/get methods ?

public class CaseInsensitiveMap extends HashMap<String, String> {
    ...
    put(String key, String value) {
       super.put(key.toLowerCase(), value);
    }

    get(String key) {
       super.get(key.toLowercase());
    }
}

This approach does not force you to change your "key" type but your Map implementation.

Answer 4

You need a wrapper class for your String key with a case-insensitive equals() and hashCode() implementation. Use that instead of the String for the Map's key.

See an example implementation at http://www.java.happycodings.com/Java_Util_Package/code3.html I found it in 2 minutes of googling. Looks sensible to me, though I've never used it.

Answer 5

The three obvious solutions that spring to mind:

• Normalise the case before using a String as a key (not Turkish locale works differently from the rest of the world).

• Use a special object type designed to be used as key. This is a common idiom for dealing with composite keys.

• Use a TreeMap with a Comparator that is case insensitive (possibly a PRIMARY or SECONDARY strength java.text.Collator). Unfortunately the Java library doesn't have a Comparator equivalent for hashCode/equals.

Answer 6

You could use my Apache licensed CaseInsensitiveMap discussed here. Unlike the Apache Commons version, it preserves the case of keys. It implements the map contract more strictly than TreeMap (plus has better concurrent semantics) (see the blog comments for details).

Answer 7

Trove4j can use custom hashing for a HashMap. This may however have performance implications given that hashcodes cannot be cached (although Trove4j may have found a way around this?). Wrapper objects (as described by John M) do not have this caching deficiency. Also see my other answer regarding TreeMap.

Answer 8

Check the accepted answer at the link below. How to check for key in a Map irrespective of the case?

Bottom line is "The simplest solution is to simply convert all inputs to uppercase (or lowercase) before inserting/checking. You could even write your own Map wrapper that would do this to ensure consistency."